<p>Machinability is a key factor for the profitability and is an elusive variable specially for automatic machining operations. Metallurgists faced with this subjective requirement have nevertheless succeeded in developing a number of modifications to the base alloys that do possess superior machinability. These modified alloys are commonly known as Free-Machining Steels or Free-Cutting Steels.</p> <p>The report commences with a general description of the mechanism by which three different types of manganese sulphide inclusions are formed in resulphurized steel. Both the selected free-machining steels have globular type of inclusions and probably are of Type I. Lead phase in leaded resulphurized steel exists in association with sulphide inclusions or free lead particles. Tests have shown that the free-machining additives
have a negligible effect on the mechanical properties of steel.</p> <p>The results on the comparative machinability of plain carbon steel and free-machining steels, under a wide range of cutting conditions, have revealed a clear difference in machinability. Free-machining steels are much superior to unmodified steels. This conclusion is based on the following criteria: tool wear, cutting forces, built up edge, shear angle, contact length, and surface finish produced.</p> <p>It is evident from the microscopic analysis of the chips produced during the cutting process that the free-machining steels have decreased chip thickness, decreased roughness
at the back of the chip, and decreased wipe-down, than the plain carbon steel. All these factors are in favour of better machinability.</p> <p>Finally, it is concluded that the leaded resulphurized steel (12L15) is superior than straight resulphurized steel (1215) below a certain critical speed. It is proposed that this critical speed is directly related to the cutting temperature, which is a function of cutting variables.</p>